This invention relates to nonleafing aluminum flake pigments and processes for making same.
Both leafing and nonleafing aluminum flake pigments are constituted of minute flakes of aluminum or aluminum-based alloys, and are typically supplied as pastes or powders for use e.g. in paints, inks and other finishes to impart metallic luster and/or silvery color to surface coatings of such finishes.
The property of "leafing" may be defined as the tendency of metal pigment flakes, when dispersed in a finish vehicle having sufficiently high surface tension (and free of substances that inhibit leafing), to become arranged in flat, parallel or overlapping relation at the surface or immediately adjacent the surface of an applied coating layer of the finish, so as to provide a highly reflective coating that simulates the appearance of bare metal. Thus, leafing aluminum pigments are those in which the proportion of flakes that "leaf" is great enough to produce this visual effect, as is desired for a variety of applications.
Nonleafing pigments (with which the present invention is concerned), on the other hand, exhibit relatively little or no characteristics of the leafing pigments. The nonleafing flakes, when dispersed in an applied coating layer of a finish, are at least predominately disposed in random attitudes and at random distances from the coating surface, providing a less reflective bare metallic appearance than when a leafing pigment is used. Nonleafing pigments are particularly desirable and effectual in thin coat application (e.g. of the order of 0.5 mils) finishes which are subject to surface abrasion, weathering or the like wherein impairment or removal of surface adjacent layers (which normally contain the leafing pigment) is highly probable and undesirable. As explained below, the leafing or nonleafing character of a pigment is determined by the use of particular materials in producing or subsequently treating its constituent flakes. Nonleafing aluminum flake pigments are generally employed in all applications when metallic luster without bare metallic appearance is desired in a coating.
The production of leafing and nonleafing aluminum flake pigments alike involves reducing particulate aluminum (such as foil scrap or atomized aluminum powder) to the desired minute flake form, conventionally by subjecting the particulate aluminum to the action of a ball mill, stamping mill or other equipment capable of flattening and breaking up the particles, in the presence of a minor proportion (based on the weight of aluminum) of a so-called "milling agent" such as a fatty acid. For convenience of reference, the operation of reducing particulate aluminum to flake form will herein be termed "milling," and the equipment in which it is performed will be termed a "mill," regardless of the specific nature of the operation and equipment used. Milling may be performed either dry (in air or other gas) or wet in a liquid hydrocarbon milling vehicle such as mineral spirits. While dry milling facilitates handling and product size classification because the metal particles can be blown into and out of the mill in dry condition, wet milling is often preferred as a precaution against explosion in view of the pyrophoric characteristics of aluminum powder. After milling, the flake particles may in some instances be subjected to various additional treatments.
Presence of the milling agent in the milling operation is essential to protect the aluminum particles during reduction so that they are flattened into flake form rather than being merely broken up, and to prevent cold welding of the particles. In addition, the milling agent covers the particle surfaces with a layer of material which remains with the particle after milling. This layer of material imparts certain characteristics to the finished flake product and protects the flake particle from corrosion, oxidation or other deleterious attack so as to aid in preserving the brightness or luster of the produced pigment, while reducing the hazard of fire or explosion incident to handling aluminum powder. The material of this layer is herein termed "milling agent residue" because it consists essentially of the milling agent compound or compounds present during milling and/or derivatives thereof produced by reaction in the course of or as a consequence of the milling operation.
It is currently believed that some milling agent residue of the described layer may actually be bonded chemically to the flake. In any event, some of the layer of residue is in very close proximity and/or direct contact with elemental metal at the flake surfaces (whether or not such contact involves chemical bonding). This is necessarily true in that the milling agent is present as the aluminum particle's elemental metal surfaces are exposed by milling.
Leafing aluminum flake pigments are made by using, as the milling agent, one or more substances herein termed "leafing milling agents" which are known to impart leafing properties to the flakes. Leafing is believed substantially attributable to the layer of residue of such milling agent on the produced flakes, and in particular to surface tension effects caused by that layer. Currently, saturated fatty acids such as stearic and/or palmitic acids are very commonly used as leafing milling agents. These acids are highly efficacious milling agents, and enable milling to be performed with high efficiency, efficiency being expressed as proportion of total particulate aluminum charge that is reduced to the desired particle size in a single pass through a mill while being flattened into flake form therein. The desired sizing of leafing flake pigments thus produced is readily controllable by, for example, variation in resident time of the charge of aluminum in the mill. Consequently, the manipulative operations involved in making leafing aluminum flakes are advantageously simple, straightforward and convenient. These manipulative operations (apart from the specific milling agent employed) will be termed "leaf milling" herein; i.e. it will be understood that the term "leaf milling" designates that milling operation (whether in a ball mill or other equipment) which, if performed with known leafing milling agents such as stearic acid, would result in production of a leafing flake pigment.
In contrast, current commercial methods of making nonleafing pigments are attended with serious difficulties and disadvantages. The pigments made in accordance with these methods have demonstrated many undesirable properties including deficiencies in one or more of the following characteristics: tinting strength (hue produced by a given volume of pigment having a specified particle size distribution in a finish), opacity (hiding ability of a given volume of pigment having a specified particle size distribution), metallic luster or brightness, flop (change in hue or lightness, e.g. color intensity, with change in viewing angle), and freedom from agglomeration and graininess.
Neither of the two procedures currently commonly employed for commercial production of non-leafing aluminum flake pigments is comparable to the ease and/or efficiency of leaf milling techniques. In one of these procedures, known as chemical deleafing, particulate aluminum is first leaf milled in the presence of a leafing milling agent (e.g. stearic and/or palmitic acid). The resultant flakes are then treated with deleafing agents such as lead naphthenate or octoate, aqueous phosphates or acetic acid. In another procedure for preparing non-leafing flake pigment, a non-leafing milling agent, such as oleic acid, is used in the milling operation.
Chemically deleafing a leafing flake pigment is disadvantageous from the standpoint of operational convenience, because it requires an additional step after milling. The deleafing agents, which remain in the product, are considered undesirable contaminants or pollutants at least for some applications. In addition, the deleafing step may sometimes tend to agglomerate the flake material.
Use of known nonleafing agents, in particular oleic acid, also presents serious drawbacks, because their effectiveness as milling agents is poor. Consequently, milling times must be short (as compared with leaf milling) to avoid product degradation. Efficiency is very low resulting in milled metal commonly containing high levels of oversized particles. This oversize may constitute as much as 30% of the total feed, necessitating successive remilling and screening to obtain a properly sized pigment. Remilling of the oversized particles is even less efficient. Use of oleic acid also apparently results in lower extent of protective covering of the flakes than is provided by leafing milling agents, and consequently increases the hazard of explosion and reduces the ability to attain very fine particle sizes. Due to this lower protection from the use of oleic acid, the production of so called dry pigments is essentially restricted to very coarse grades. The production of dry pigments, i.e. those containing very low quantities of the liquid milling vehicle, is highly desirable in situations wherein the liquid vehicle is deleterious to the particular pigment application.
Currently commercially available nonleafing aluminum flake pigments, as produced by the above described procedures, have substantial shortcomings. Chemically deleafed pigments, even if not agglomerated, are dull and relatively unattractive, possibly owing to etching of the flake surfaces by the deleafing agent. Pigments made with oleic acid as a milling agent tend to form insoluble agglomerates, especially in the finer grades, upon standing for any length of time, with resultant undesired graininess. Indeed, progressive agglomeration often renders them virtually unuseable after relatively short periods of storage. Known additives for inhibiting such agglomeration have adverse effects in various applications in which the pigments may be used. Moreover, conventional nonleafing milling agents such as oleic acid afford poor particle size control, which detracts from such desired product qualities as consistent tinting strength and opacity.
Alternative production of nonleafing aluminum flake pigments have been set forth in U.S. Pat. Nos. 2,858,230 (treatment of leafing pigment wich an aqueous solution containing available PO.sub.4 ion), 3,264,129 (use of certain aliphatic amines as milling agents), and 3,389,105 (use of fluorocarbon resins as milling agents). These proposals, however, have not found commercial acceptance as ways to overcome the problems associated with current nonleafing aluminum flake pigments and their manufacture.
U.S. Pat. No. 3,781,177 teaches that admixture of isostearic acid with previously milled aluminum flake powder (presumably already bearing a layer of milling agent residue) agglomerates and thereby dedusts the powder for explosive use. Such agglomeration is undesirable for pigments.